"From darkness to radiance": Light-induced type I and II ROS-mediated apoptosis for anticancer effects of...

Photodynamic therapy relies on the generation of cytotoxic effects triggered by the irradiation of a photosensitizer molecule, resulting in the production of reactive oxygen species at concentrations exceeding physiological levels. In this context, squaraine dyes, a prominent family of second-generation photosensitizers, have gained increasing attention for their remarkable properties, with their photobiological characteristics recently emerging as a key focus of in-depth research. Dansylpiperazine-bearing squaraine dyes exhibited strong absorption in the red visible spectral region, excellent photostability, and a predicted ability to interact with human serum albumin, potentially serving as a transport vehicle to target sites. Benzothiazole derivatives excelled in photodynamic activity, demonstrating 7- to 11-fold increased cytotoxicity upon irradiation against prostate adenocarcinoma PC-3 cells and tumor selectivity indices exceeding 10 when compared to normal NHDF cells. In contrast, the introduction of the dansylpiperazino group in indole-derived compounds unexpectedly declined their photodynamic activity. Concerning benzothiazole-based ones, multiple reactive oxygen species were shown to contribute to the photodynamic effects, with singlet oxygen playing a key role. Squaraine internalization was observed in various cytoplasmic organelles, including mitochondria, endoplasmic reticulum, and lysosomes, without clear evidence of preferential localization to any single organelle. Non-genotoxic in the dark, the squaraines induced cell death by apoptosis upon light activation, as evidenced by significant DNA fragmentation and increased caspase 3/7 activation, particularly for the dye with N-ethyl chains, at concentrations below 1.0 μM, underscoring their potency. Checkpoint arrests in G1 and G2/mitosis were observed for non-irradiated and irradiated conditions, respectively, highlighting the antiproliferative effects of these squaraine dyes.